1. Field of the Invention
The invention relates to fault detection apparatus for digital displays particularly with respect to seven segment numeric displays having a capacitive characteristic, such as liquid crystal displays (LCD) and electro luminescent displays (EL).
2. Description of the Prior Art
The replacement of numerical mechanical displays by digital electronic displays in industrial applications, such as avionics, has engendered a significant reliability problem. The problem is especially severe where the information is safety critical to, for example, aircraft operating crews and passengers or to the process being controlled. Each digit of such displays is typically embodied by an array of seven segments arranged and operated to selectively display the decimal digits zero to nine. Four of the segments are oriented vertically as spaced vertical pairs and the remaining three segments are disposed horizontally at the top, center, and bottom of the digit array. The segments of such displays are typically implemented by light emitting diodes (LED), gas discharge elements, liquid crystal elements (LCD), electro luminescent elements (EL) and the like.
Most failures where a segment is either erroneously on or erroneously off result in characters that will appear to the user as obviously non-numeric. There are, however, fifteen single segment failures which result in non-obvious faults where a decimal digit is displayed other than the digit commanded. Thus, erroneous information is displayed to the user which can precipitate a dangerous situation. For example, such faults are critical in an avionics altitude indicator which instead of displaying the commanded altitude of 3000 feet, erroneously displays 9000 feet.
A prior art solution to the reliability problem existing with such displays is to provide a user initiated push to test configuration where the results are verified by the user. The push to test verification procedure may, for example, provide a flashing display of figure eights where energization is continuously applied and removed from all of the segments. Erroneously on and off segments may be detected by this procedure. Probability analysis utilizing the failure rate of the components combined with exposure time determines the maximum time interval between tests. Such user initiated tests suffer from the disadvantage that human judgement is utilized to verify the validity of segments and hence the test is subject to human error. Additionally, human error may result in the test not being performed within the specified time intervals. Such testing also fails to provide maintainence monitoring data for retrieval by maintainence personnel.
A prior art example of apparatus for providing digital display segment verification is disclosed in U.S. Pat. No. 4,307,392 issued Dec. 22, 1981. The apparatus of said U.S. Pat. No. 4,307,392 is particularly designed for LED type displays and detects failure by a segment voltage level measurement. Such apparatus cannot be utilized with a liquid crystal type display because liquid crystal segments exhibit a capacitive nature and, therefore, cannot be verified by voltage level measurements. The apparatus of said U.S. Pat. No. 4,307,392 utilizes parallel data transmission and thus requires a significant amount of interconnection circuitry to the control unit. Such a parallel arrangement renders remotely locating the control unit with respect to the display difficult and impractical. Other prior art segment verification apparatus utilize current level measurements which also cannot determine segment status of capacitive segments such as in liquid crystal displays.
Another system for monitoring the status of segments in a digital display is disclosed in U.S. Pat. No. 4,247,852 issued Jan. 27, 1981. The apparatus of said U.S. Pat. No. 4,247,852 is designed for liquid crystal displays and measures the voltage level on a test lead connected to the segment. Voltage level comparisons between energizing leads and test leads provide the error status signal. Since the LCD segments are capacitive it is believed that such voltage level measurements will not provide effective error determination for many types of commonly occuring faults. For example, the failure of a segment driver to a high or low state or a segment to ground short may not be detectable by the apparatus of said U.S. Pat. No. 4,247,852. The requirement of an additional test lead for each segment of the LCD significantly increases the complexity thereof as well as the complexity of the associated electronic circuitry. The additional test lead per segment also precludes the utilization of commercially procureable LCD indicators which are generally not provided with such test leads.
None of the prior art apparatus provides a practical system for electronically verifying the operation of digital displays with segments of a capacitive nature. Segment status determination is effected by measuring a voltage level or a current level. Displays of a capacitive nature, such as LCD's, cannot effectively be verified by voltage level or current level measurements. The prior art apparatus requiring human intervention to initiate or verify the proper operation of all of the segments are subject to human error. Thus, the prior art does not provide effective automatic self testing apparatus for digital displays having segements of a capacitive nature.